Wednesday
QuickCheck 30.7
Both segments of the wire are
made of the same metal. Current
I1 flows into segment 1 from the
left. How does current I1 in
segment 1 compare to current I2 in
segment 2?
A.
B.
C.
D.
I1 > I2.
I1 = I2.
I1 < I2.
There’s not enough information to compare them.
Slide 30-60
QuickCheck 30.7
Both segments of the wire are
made of the same metal. Current
I1 flows into segment 1 from the
left. How does current I1 in
segment 1 compare to current I2 in
segment 2?
A.
B.
C.
D.
I1 > I2.
I1 = I2.
Conservation of current
I1 < I2.
There’s not enough information to compare them.
Slide 30-61
QuickCheck 30.9
Both segments of the wire are made of
the same metal. Current I1 flows into
segment 1 from the left. How does the
electric field E1 in segment 1 compare
to the electric field E2 in segment 2?
A.
B.
C.
D.
E.
E1 > E2.
E1 = E2 but not zero.
E1 < E2.
Both are zero because metal is a conductor.
There’s not enough information to compare them.
Slide 30-64
QuickCheck 30.9
Both segments of the wire are made of
the same metal. Current I1 flows into
segment 1 from the left. How does the
electric field E1 in segment 1 compare
to the electric field E2 in segment 2?
A.
B.
C.
D.
E.
E1 > E2 .
J = σE
E1 = E2 but not zero.
E1 < E2.
Both are zero because metal is a conductor.
There’s not enough information to compare them.
Slide 30-65
Consider another case.
What does this tell us about the surface
charge density between 1 and 2?
Recap
dQ
I
dt
• Current:
macroscopic
• Current density:


J  E
ne e 
J
E
m
2
microscopic
Resistivity and conductivity
• Conductivity:
• Resistivity:
ne e 

m
2
Conductivity and Resistivity
Slide 30-67
Conductivity and Resistivity
This woman is measuring her percentage body fat by gripping a device that
sends a small electric current through her body. Because muscle and fat
have different resistivities, the amount of current allows the fat-to-muscle
ratio to be determined.
Slide 30-66
Superconductivity
 In 1911, the Dutch physicist
Kamerlingh Onnes discovered
that certain materials suddenly
and dramatically lose all
resistance to current when
cooled below a certain
temperature.
 This complete loss of resistance
at low temperatures is called
superconductivity.
Superconductors have unusual magnetic
properties. Here a small permanent magnet
levitates above a disk of the high temperature
superconductor YBa2Cu3O7 that has been
cooled to liquid-nitrogen temperature.
Slide 30-69
Resistance and resistivity
Which has the largest resistance?
Which has the smallest resistance?
Current and voltage
A
I  JA  AE 
V
l
Ohm’s Law
Slide 22-25
The current through a wire is measured as the potential
difference V is varied. What is the wire’s resistance?
A. 0.01 .
B. 0.02 .
C. 50 .
D. 100 .
E. Some other value.
QuickCheck 30.11
The current through a
wire is measured as the
potential difference V
is varied. What is the
wire’s resistance?
A.
B.
C.
D.
E.
0.01 .
0.02 .
50 .
100 .
Some other value.
Slide 30-77
Nonohmic Materials
 Some materials and devices are nonohmic,
meaning that the current through the device is not
directly proportional to the potential difference.
 Diodes, batteries, and capacitors are all nonohmic
devices.
Slide 30-78
Recap:
• .
 1 ohm  1   1 V/A.
L
A Basic Battery Circuit: The electric potential in
the circuit is increased by the battery
See also Lab Week 4
Slide 22-3
The Basic Circuit
 The most basic electric circuit
is a single resistor connected
to the two terminals of a
battery.
 Figure (a) shows a literal
picture of the circuit elements
and the connecting wires.
 Figure (b) is the circuit
diagram.
 This is a complete circuit,
forming a continuous path
between the battery
terminals.
Slide 31-36
Images from : http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir2.html
A wire connects the positive
and negative terminals of a
battery. Two identical wires
connect the positive and
negative terminals of an
identical battery. Rank in order,
from largest to smallest, the
currents Ia to Id at points a to d.
A.
B.
C.
D.
E.
Ic = Id > Ia > I b
I a = I b > Ic = Id
I c = I d > I a = Ib
Ia = Ib = I c = I d
Ia > Ib > I c = I d
A wire connects the positive
and negative terminals of a
battery. Two identical wires
connect the positive and
negative terminals of an
identical battery. Rank in order,
from largest to smallest, the
currents Ia to Id at points a to d.
A.
B.
C.
D.
E.
I c = I d > I a > Ib
Ia = Ib > I c = I d
Ic = Id > Ia = I b
I a = I b = Ic = Id
I a > I b > Ic = Id
A comparison
Images from : http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir2.html
The water flow at position R is 5 cm3/s
How large is the current
A) through the straight pieces of pipe?
B) through the pump?
Conservation of Liquid
The current through the resistor R is 5A.
How large is the current
A) through the horizontal wires?
B) through the battery?
Conservation of Charge
A comparison
Images from : http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir2.html
What does the pump create in the flow
of water?
Pumps create pressure difference.
You can pump water uphill.
What does the battery establish in the
electric circuit?
A battery creates a potential difference
in a circuit.
Power Dissipation in a Resistor
A current-carrying resistor dissipates power because
the electric force does work on the charges.
Slide 31-45
Energy and Power
 The power supplied by a battery is:
 The units of power are J/s or W.
 The power dissipated by a resistor is:
 Or, in terms of the potential drop across the resistor:
Slide 31-43
Units of Energy
• Electron-volt (eV) = amount of energy it takes to move
1 electron through a potential difference of 1 volt
• Kilowatt-hour: what you see on your electricity bill!
• A load that consumes P kw in t hours, has consumed Pxt
kilowatt-hours of energy
A resistor R has current I running through it. What
happens to the power dissipated by the resistor when the
current is doubled?
A. Doubles (x2)
B. Quadruples (x4)
C. Stays the same
D. Halves (x0.5)
E. Something else?
R
I
• A resistor R has current I running
through it. What happens to the
power dissipated by the resistor
when the current is doubled?
A. Doubles (x2)
B. Quadruples (x4)
C. Stays the same
D. Halves (x0.5)
E. Something else?
R
I
Which has a larger resistance, a 60 W lightbulb or a 100
W lightbulb?
A. The 60 W bulb.
B. The 100 W bulb.
C. Their resistances are the
same.
D. There’s not enough
information to tell.
QuickCheck 31.8
Which has a larger resistance, a 60 W lightbulb or
a 100 W lightbulb?
A.
B.
C.
D.
The 60 W bulb.
The 100 W bulb.
Their resistances are the same.
There’s not enough information to tell.
Slide 31-49